Cell fate maps describe how the sequence of cell division, migration, and apoptosis transform a zygote[unreadable] into an adult and are fundamental to understanding stem cell biology. Yet, it is only in the transparent worm C.[unreadable] elegans where tedious microscopic observation of each cell division has allowed for construction of a complete[unreadable] cell fate map. More complex?and opaque?animals prove less yielding. DNA replication, however, inevitably[unreadable] generates somatic mutations. Consequently, multicellular organisms comprise mosaics where most cells[unreadable] acquire unique genomes that are potentially capable of delineating their ancestry. We propose to construct[unreadable] mammalian cell fate maps using a phylogenetic approach to passively retrace embryonic relationships by[unreadable] deducing the order in which mutations have arisen during development. We have found that polyguanine[unreadable] repeat DNA sequences are particularly useful genetic markers, because they frequently change length during[unreadable] mitosis. To demonstrate feasibility, we have used phylogenetics to reconstruct the lineage of cultured mouse[unreadable] NIH3T3 fibroblasts based on mutations affecting the length of polyguanine markers. We have then employed[unreadable] whole genome amplification to genotype polyguanine markers in single cells taken from a mouse and used[unreadable] phylogenetics to infer the developmental relationships of the sampled tissues. Our preliminary results[unreadable] demonstrate the potential of this approach for retrospectively producing a complete mammalian cell fate that,[unreadable] in principle, could describe the developmental lineage of any cell and resolve outstanding questions relevant to[unreadable] stem cell biology. [unreadable]